In connection with soils, clay may be defined, in the most general terms, as being the substance which imparts plasticity and adhesiveness to soils when wetted and kneaded, and which, when heated to redness, loses this property completely and permanently, becoming hard and coherent in proportion to the degree of heat to which it is exposed.
In common life, however, the name is applied to the whole of any naturally occurring earth which on wetting and kneading assumes a reasonable degree of plasticity and adhesiveness. When the latter property becomes nearly or quite insensible, the earth is designated as a “loam,” more or less “clayey” according to the amount of the pure, plastic and adhesive material associated with the mineral powders and sand that form the bulk of most soils.
Chemically, the pure clay substance[18] probably consists (as has been stated above) of silica and alumina in the proportion of nearly 46 to 40, the rest (14%) being water of hydration, which is lost on burning the clayey material. But while it is true that such is the composition of the plastic substance of clays, plasticity and adhesiveness are by no means invariable properties of this compound. In its purest state, as kaolinite, it is readily mistaken for chalk, (and is sometimes used as such), being powdery to the touch and entirely devoid of plasticity[19] when wetted and kneaded. The microscope shows this chalky kaolinite to consist of minute, mostly rounded, originally six-sided, thin plates, which when pure resemble to the touch powdered talc (soapstone) or even black-lead, rather than any clay known to common life. But being exceedingly soft, the kaolinite substance is easily ground or triturated into an extremely fine powder; and Johnson and Blake[20] succeeded in producing sensible plasticity and adhesiveness by long-continued trituration of kaolinite with water in a mortar. A similar process, but continued much longer by the mechanical agencies concerned in soil-formation ([see chapt. I]), is unquestionably the chief factor concerned in the formation of natural plastic clays; but whether this is the only process by which the powdery kaolinite may be transformed into plastic clay, is a question not definitely settled. It is at least possible that repeated freezing and thawing, as well as the action of hot water, may take a part in the transformation, beyond that by which they destroy the crumbly (flocculated) structure of soils and clays, and render them plastic; as is done in the maturing of clays by potters.
Causes of Plasticity.—In any case the property of plasticity and adhesiveness is restricted to the particles so fine that they fail to settle, in the course of 24 hours, through a column of pure water eight inches (200 m) high, while some are so extremely minute that they will not settle for many months, and even for several years.[21] Such turbid “clay water” may sometimes be found existing in nature, in moist, secluded places, for weeks after the subsidence of the overflows of rivers whose water is exceptionally free from dissolved mineral matter.
Separation of Colloidal Clay.—This property of the plastic clay substance, of diffusing in pure water, furnishes the means of separating from it the coarser, sandy and silty portions of soils and natural clays, and observing its characteristic properties, so far as the almost unavoidable admixture of some other substances, presently to be considered, permits.
In natural soils the clay particles usually incrust the powdery ingredients, cementing them together; or themselves form complex aggregates (floccules) of large numbers of individual particles. These may be loosened from their adhesion or cohesion either by prolonged, gentle kneading of the wet clay, or by more or less prolonged digestion (soaking) in hot water, or more expeditiously, by lively boiling with water. The boiling should not, however, be prolonged beyond the time actually required for disintegration, since (as Osborne[22] has shown) long-protracted boiling tends to render the clay permanently less diffusible.
From the turbid clay-water the diffused clay may be obtained either by evaporating the water (which as the bulk is very large, is usually inconvenient), or, more conveniently, by throwing it down from its suspension by the action of certain substances which possess the property of curdling (coagulating) the clay substance into flocculent masses that settle quickly. Of all known substances, lime, in the form of lime-water, acts most energetically in producing this change; but other solutions of lime, as well as most salts and mineral acids, produce the same effects when used in sufficient quantity. Common salt is among the most convenient, because it can most readily be leached out of the clay precipitate thus thrown down. This when white, resembles boiled starch, but being usually colored by iron might be easily mistaken for the mixed precipitate of ferric hydrate and alumina so commonly obtained by chemists in soil analysis. When separated from the water and dried, the jelly-like substance (“colloidal clay”) shrinks as extravagantly as would so much boiled starch, into hard, shiny crusts or flakes, which when struck in mass are sometimes even resonant, and bear more resemblance to glue than to the clay of everyday life. Like glue, too, but much more quickly and tenaciously, the dried colloidal clay adheres to the tongue, so as to render the separation painful; when wetted it quickly bulges with great energy, and in a short time resumes its former jelly-like condition. When moistened with less water it assumes a highly plastic and adhesive condition, so that it is difficult to handle and almost as sure to soil the operator’s hands as so much pitch.
Effects of Alkali Carbonates upon Clay.—The carbonate of potash and soda, when in very dilute solution (.01 to .05%) exert upon diffused clay an effect the reverse of the acids and neutral salts. They destroy the flocculent aggregates formed by precipitation with these, or naturally existing in the soil, and tend to puddle the clay so as to render it impervious to water. It is thus that in the alkali lands of the arid regions we often find the soil or subsoil consolidated into a very refractory “hardpan,” difficult to break even with a sledge hammer and impossible to reduce to tilth until the alkali carbonate is destroyed by means of a lime salt, such as gypsum. ([See chapt. 23]). Ammonia water also helps to cause the diffusion of clay in water, but its effect of course disappears upon drying. It is probable that this property of sodic carbonate can be utilized in rendering earth dams firmer and more secure against the penetration of water.